The present invention relates to the detection, by a receiver, of signal bursts transmitted on a radio channel in a communication system.
It finds an application in particular in the detection of short bursts sent in a radiocommunication network. These bursts may be of various types, such as initial-synchronization bursts or bursts of random access to the mobile network. The latter case will be more particularly developed hereinbelow, without this being limiting.
When a mobile terminal of a communication network wishes to avail itself of communication resources, for example to make a call, it executes a request to the network that manages and distributes the resources. This random access request generally consists in the transmission of a message whose preamble is a signal burst representing a predetermined digital sequence. This message is sent on an uplink radio channel listened to by a reception system of the network. In radiocommunication systems such as GSM (“Global System for Mobile communications”) and UMTS (“Universal Mobile Telecommunication System”), this channel is called RACH or PRACH (“Packet Random Access CHannel”). The format of such a message is in particular described in section 5.2.2.2 of the TS 25.211 technical specification version 5.2.0 Release 5, “Physical channels and mapping of transport channels onto physical channels (FDD)”, published in September 2002 by the 3GPP organization.
Reliable detection of random access bursts on the RACH is important since the communications setup failure rate seen by a mobile radio user depends directly thereon.
An improvement in the reliability of detection is particularly beneficial in respect of reception systems that comprise sectorial or omnidirectional smart antennas.
In the UMTS system, the predetermined digital sequence sent on the RACH channel by a mobile terminal has a size of 4096 “chips”, a chip being an element of code in accordance with the coding used in the system. The data exchanged consist of 10 ms frames, themselves subdivided into 15 time intervals (or “slots”) of 666 μs, corresponding to 2560 chips. Thus, the signal burst associated with the digital sequence sent on the RACH is received within an interval corresponding to two consecutive slots.
When the radio network wishes to determine whether a random access burst has been transmitted on an RACH channel, it calculates for the 1024 (=2×2560−4096) possible positions of the digital sequence of the burst within two consecutive slots, a correlation between the sequence as detected and the predetermined digital sequence which is known to the network.
A criterion must be defined to decide, on the basis of such a correlation, whether the predetermined digital sequence is present. This criterion is customarily based on the correlation's energy level which is compared with a predefined threshold level.
However, depending on the propagation conditions of the radio channel used, the signal received by the radio network is attenuated to a greater or lesser extent. It follows that the fixing of the threshold is tricky: too low a threshold gives rise to numerous false detections that disturb the system, whereas too high a threshold causes access requests originating from terminals relatively far from the base station to be missed.
A power ramp can be used by the mobile terminal to regularly retransmit the burst for access to the network on the RACH channel, with increased transmission power for each new transmission, for as long as the network has not responded to its request for resources. This method makes it possible to improve the detection of the burst by the radio network, in particular in the case where the low transmission power of the first transmissions is the reason for the absence of detection of the burst on the RACH.
However, through the repetition of the random access burst on the RACH, this method occupies the channel to the detriment of any requests from the other users. Furthermore, the high power of the signals thus repeated may create nuisance interference in the system.
An object of the present invention is to propose a method for detecting predefined signals which makes it possible to attenuate the drawbacks of the known methods.
Another object of the invention is to enhance the consideration of the propagation conditions on the radio channel so as to improve the detection of the transmitted bursts.